1. Field of the Invention
The present invention relates to a method for manufacturing a plastic foam, and more precisely it relates to a method for manufacturing a plastic foam by a dielectric heating.
2. Description of the Related Art
Various methods for manufacturing a plastic foam are known. The assignee of the present application has proposed an improved method for manufacturing a plastic foam in Japanese Unexamined Patent Publication (Kokai) No. 57-170727 in which a plastic material having a polar group, such as polyvinyl chloride or the like, which has a specific electrical property of a large dielectric loss factor (tan.delta.) is premixed with a chemical foaming agent which produces a gas when it decomposes, and the mixture is placed in a predetermined shape of a molding cavity defined by a pair of dies with electrodes.
The mixture is then subjected to dielectric heating to melt the polyvinyl chloride (dielectric) or the like and to decompose the chemical foaming agent in order to obtain a formed product having a predetermined shape.
In the manufacturing method disclosed in the aforementioned publication, a large dielectric loss of the dielectric, such as polyvinyl chloride having a polar group takes place in an alternating current electric field, particularly in a high frequency electric field, so that the dielectric produces heat. Accordingly, when the plastic material such as polyvinyl chloride is put in the molding cavity of a pair of dies which is made of a material having a small dielectric loss to apply a high frequency voltage to the plastic material through the molding dies, the polyvinyl chloride or the like is heated and becomes molten, so that the plastic material is deformed into a predetermined shape corresponding to the shape of the molding cavity. Namely, the manufacturing method is characterized by the utilization of a self-exothermic property of the dielectric, such as polyviyl chloride. In other words, the manufacturing method mentioned above is clearly distinguished from a conventional compression molding in which molding dies are heated by an outside heater, so that a material in a molding cavity can be heated by the heat transmitted to the molding dies to form a predetermined shape of mold products. Accordingly, the molding method disclosed in the publication mentioned above has a higher heat efficiency than the conventional compression molding, and in particular a center portion (core) of the plastic material can be effectively heated, in comparison with a circumferential portion thereof.
A plastic foam obtained by the improved manufacturing method disclosed in the prior application has a core consisting of highly expanded cellular member and an outer or a skin shell consisting of high density layer, since the foaming agent contained in the skin shell is decomposed with the liberation of gas, by application of a high frequency voltage and the center portion of the plastic material is more effectively heated, and expanded as mentioned above. With the cellular construction mentioned above, the plastic foam can be advantageously used to form moldings for automobiles, such as decorative and protective elongated moldings which are applied to side panels and door panels of the automobiles and which must be light, decorative and shock-absorptive.
In the molding process mentioned above, a volumetric expansion of the polyvinyl chloride material due to the decomposition of the chemical foaming agent takes place. Accordingly, it is necessary to provide an air gap between the plastic material and a cavity surface of the upper die in the molding cavity in order to accomodate the expansion. However, it is known that such an air gap between the raw material, such as polyvinyl chloride which is charged in a lower molding die and an upper molding die to accomodate the expansion, decreases the actual voltage applied to the raw material in comparison with the voltage which is produced by and between the electrodes.
It should be recalled that a heat value of an object to be heated by the dielectric heating (the foaming raw material, such as polyvinyl chloride resin in the embodiment of the present invention) is in proportion to a square of the voltage actually applied thereto. Therefore, in a method which provides an air gap between the raw material charged in a lower die and a cavity surface of the upper die, the foaming raw material can be slowly heated, supposing that the voltage between the electrodes and the frequency thereof are constant. This results in a long time for heating the raw material to a predetermined temperature which is necessary for melting the material and for decomposing the foaming agent contained therein.
It is, therefore, necessary to apply a higher voltage to the foaming raw material in order to reduce the heating time. An increased voltage, however, raises another problem particularly in a dielectric heating molding method mentioned above in which an air gap which tends to become large in order to produce a plastic foam having a higher foaming ratio is provided between the foaming raw material charged in the lower die and the upper die. Namely, since the intensity of the electric field in the air gap is getting large in proportion to the increase of the voltage applied to the electrodes, there is a possibility that an electric discharging takes place between the surface of the foaming raw material and the upper die. This makes it impossible to largely increase the voltage between the electrodes.
Under these circumstances, it is preferable not to provide an air gap or to provide a slight air gap between the foaming raw material and the upper die. The absense of the gap or the slight gap, however, restricts the foaming ratio to a small value, which is contrary to a desire to produce plastic foam products having a high foaming ratio.
As mentioned before, moldings for automobiles must be light and highly decorative and protective. In general, the moldings for automobiles are shaped to have opposite ends of a stream line shape like a projectile and different sectional shapes in their longitudinal directions mainly for the purpose of decoration. Such highly decorative and light moldings having a high foaming ratio can not be produced by the prior art method mentioned above.
The primary object of the present invention is, therefore, to provide an improved method for manufacturing a plastic foam which has a large foaming ratio in a dielectric heating process.
Another object of the present invention is to provide a novel method for manufacturing a plastic foam in a dielectric heating process, with an increased energy transmission efficiency.
Still another object of the present invention is to provide a method for manufacturing a predetermined shape of plastic foam, in which not only a preformed foaming material having a foaming agent mixed therewith but also a powdery, granular or liquid foaming material having a foaming agent mixed therewith can be used as a raw material.
Still another object of the present invention is to provide a method for manufacturing a plastic foam which has a core having a large foaming ratio and a dense or solid outer shell having a small foaming ratio.
In order to achieve the objects mentioned above, according to the present invention, there is provided a method for manufacturing a predetermined shape of plastic foam, wherein a plastic compound with a chemical foaming agent is charged in a molding cavity which is defined by a pair of opposed split cavity surfaces and the plastic compound is subject to a dielectric heating to plasticize and melt it and to decompose the chemical foaming agent, characterized in that one of the cavity surfaces is movable relative to the other cavity surface to vary the volume of the molding cavity, in accordance with the expansion of the plastic compound in the molding cavity during heating, so that the plastic compound is kept in contact with the cavity surfaces without an air gap between the plastic compound and the cavity surfaces during the expansion of the plastic compound due to the dielectric heating.
The movement of the movable cavity surface is effected either by the expansion of the plastic compound or by an actuator operatively connected to the movable cavity surface.
According to the manufacturing method of the present invention as mentioned above, a high frequency voltage is applied to a dielectric (object) to be heated which is located between the electrodes. It should be born in mind that the heat value of the dielectric object is in proportion to a square of the voltage which is actually applied to the dielectric, and the actual voltage applied to the dielectric is approximately in inverse proportion to a distance of the air gap, if any, between the dielectric and the electrodes, so that the heat value becomes small as the air gap increases. Since one of the cavity surfaces of split type molding dies having high frequency electrodes is movable, the plastic compound with a foaming agent mixed therein is charged in the molding cavity and held between the cavity surfaces of the molding dies, without a gap between the electrodes exactly speaking, the cavity surfaces and the plastic compound. With this arrangement, the voltage produced by and between the electrodes can be substantially directly applied to the plastic compound to be heated, so that the plastic compound is molten and plasticized. When the plastic compound is plasticized, the chemical foaming agent incorporated in the plastic compound decomposes and produces a gas to expand the plastic compound. By retracting the movable cavity surface in accordance with the expansion of the plastic compound, the plastic compound can be substantially kept in surface contact with the cavity surfaces, so that the high frequency voltage continues to be effectively applied to the plastic compound. This results in a production of a predetermined shape of molded product in a short space of time. Since the plasticization and the expansion of the plastic compound take place as a result of a self-exothermic function due to the dielectric operation of the high frequency voltage, the molded product has a core having a larger foaming ratio in comparison with the remaining portion thereof.
Preferably, the movable cavity surface continuously receives a back pressure acting in the opposite direction, so that it is retracted against the back pressure so as to come into contact with the plastic compound charged in the molding cavity. This enables the cells of the outer shell of the foaming plastic compound to be destroyed by the expansion pressure during the expansion thereof, so that the solid outer shell has a small foaming ratio.
When a granular or powdery plastic foaming raw material (plastic compound) is used, the air between the grains or particles is discharged therefrom by the gas produced by the chemical foaming agent at the initial stage of the decomposition of the foaming agent, so that the spaces between the grains or particles contribute to a developed formation of the cell construction of the product.
Thus, according to the present invention, the molded products have a larger foaming ratio at their cores and have an increased foaming ratio as a whole, in comparison with the molded products obtained by the prior art.